Gas hydrates are solid structures composed of gas molecules encased in cages of water molecules. Most interesting to the energy community are hydrates that contain hydrocarbon gases. In the northern Gulf of Mexico, hydrates of this type form in water depths greater than 450 m and often occur in mounds located where faults intersect the seafloor. Typically, these hydrates consist of seawater from which the salt has been excluded and gases that have migrated up faults from buried hydrocarbon reservoirs. In addition to hydrates, the mounds contain large amounts of calcium carbonate and various other minerals precipitated by microbes that extract energy from hydrocarbon fluids.